Work machines such as, for example, wheel loaders, track type tractors, and other types of heavy machinery are used for a variety of tasks. These work machines include a power source, which may be, for example, an engine such as a diesel engine, a gasoline engine, or a natural gas engine, that provides the power required to complete these tasks. To efficiently perform these tasks, the work machines require a transmission that is capable of transmitting the torque generated by the engine over a wide range of speeds.
Typically, work machines use a continuously variable transmission (CVT) to convert engine torque to drive traction devices, such as wheels or tracks that propel the work machine. Continuously variable transmissions are capable of providing a desired output torque at any speed within its operating range by continuously changing the ratio of the transmission.
When an increase in torque is required at the same or faster output speed than previously demanded, a power increase demand is placed on the engine. Similarly, when less torque is required at the same or slower speed, a power decrease demand is transmitted to the engine. A change in the engine power demand is traditionally countered by an increase or decrease in fuel delivery to the engine. However, due to response delays of the various engine systems and the immediate demand for a change in power, engine speed may either droop under or over shoot a desired engine speed required to meet the output demand.
A problem common to many known CVT systems is that the operation of these devices may produce loads on the engine that are severe enough to cause engine “stalling” or “lugging”, a.k.a, excessive engine speed droop. “Lugging” or “stalling” the engine may decrease the productivity and efficiency of the engine. Such CVT systems may also become unstable because the time required for the engine to respond to the changes in power demand can be much greater than the demand period.
Traditionally, power systems including an engine and a continuously variable transmission are controlled by measuring engine speed and changing the ratio of the transmission to keep the engine within a defined speed range. For example, U.S. Pat. No. 6,385,970 to Kuras et al. discloses a system that includes an engine, a hydraulic continuously variable transmission, and a control system in communication with the engine and transmission. The control system of the '970 patent is an under-speed control system for a hydro-mechanical drive system that is operable to sense engine speed and create an output speed signal. The control system is further operable to compare the engine speed signal to an under-speed value and produce an error signal. The error signal is used to produce a command signal that controls the transmission ratio so as to manage the load on the engine.
However, this type of control system may not prevent the engine from experiencing the inefficiencies associated with engine over-speed or under-speed conditions. Because the control system attempts to maintain the desired engine speed by measuring a deviation of the actual speed from a desired speed, the actual engine speed may not match the desired engine speed. By the time the control system determines that the engine speed has deviated from the desired speed, the engine may have already experienced these inefficiencies, even if the transmission adjusts the ratio to help the engine recover.
The present invention is directed towards overcoming one or more of the problems as set forth above.